Common face-up optical semiconductor devices can be configured to include various semiconductor layers (including an n-type semiconductor layer formed on an insulative growth substrate, and an active layer serving as a light emitting layer, and a p-type semiconductor layer), a transparent electrode layer, a p-side electrode formed on the transparent electrode, and an n-side electrode formed on an exposed portion of the n-type semiconductor layer. Here, the transparent electrode layer can be formed on an entire remaining p-type semiconductor layer after the p-type semiconductor electrode and the active layer are partly removed. In the thus-configured semiconductor device, the transparent electrode can have a function to even the light emission distribution by uniformly diffusing current flow across the entire optical semiconductor device. When the transparent electrode is simply designed without intended purpose, however, the current may concentrate near a straight line connecting the p-side electrode and the n-side electrode with the shortest distance at the center. This means the light emission distribution may be concentrated on and near this line, resulting in uneven light emission distribution.
FIG. 1 corresponding to FIG. 5 of Japanese Patent Application Laid-Open No. 2005-039264 is a top view showing a conventional face-up optical semiconductor device. The device of FIG. 1 has a square shape when viewed from above.
As shown in FIG. 1, an n-type semiconductor layer 102, an active layer 103 serving as a light emission layer, and a p-type semiconductor layer 104, are formed on a growth substrate 101. Also after removing part of the p-type semiconductor layer 104 and the active layer 103, a transparent electrode layer 105 is formed on the entire surface of the remaining p-type semiconductor layer 104 except for the outer peripheral portion of the optical semiconductor device. A p-side electrode 106 is formed on the transparent electrode layer 105 so as to be composed of an electrode seating portion 1061 (hereinafter, referred to as “pad portion”) and secondary extending portions 1062a and 1062b (hereinafter, referred to as “auxiliary electrode portion”). An n-side electrode 107 is formed on an exposed portion of the n-type semiconductor layer 102. The distance between the p-side electrode 106 and the n-side electrode 107 is designed to be approximately constant, thereby causing a current flowing from the p-side electrode 106 to the n-side electrode 107 to be uniformly diffused as shown by the solid arrows in FIG. 1. In this case, if the distance between the p-side electrode 106 and the n-side electrode 107 is smaller than a current diffusion distance D which will be described later, the current flowing from the p-side electrode 106 to the n-side electrode 107 can reach the end of the transparent electrode layer 105 near the n-side electrode 107. Accordingly, the entire semiconductor layers disposed therebetween, in particular, the entire active layer 103, can be evenly supplied with a current, thereby evening the light emission distribution.
FIG. 2 is a top view of a second conventional face-up optical semiconductor device. The shape of the face-up optical semiconductor device of FIG. 2 is a rectangle. This is because such a recent type of optical semiconductor device can be often installed in a thin light source device for backlight purpose and the optical semiconductor device may be designed to have a rectangular top shape in order to conform to the shape of the thin light source device. It should be noted that a direction parallel to the short side of the rectangle is referred to as a short side direction whereas a direction parallel to the longer side of the rectangle is referred to as a longitudinal direction.
In FIG. 2, as in FIG. 1, an n-type semiconductor layer 102, an active layer 103, a p-type semiconductor layer 104, a transparent electrode layer 105, and an n-side electrode 107 are formed on a growth substrate 101. The p-side electrode 106′ can be composed only of a pad portion. (See Japanese Patent Application Laid-Open No. 2008-4729, in particular, FIG. 1.) Namely, the auxiliary electrode portions 1062a and 1062b in FIG. 1 are not formed in this semiconductor device. In this configuration, if the p-side electrode 106′ and the n-side electrode 107 can be configured to be made large with respect to the short side width, the current can be evenly diffused in the short side direction to a certain extent.
However, as shown in FIG. 2, the distance between the p-side electrode 106′ and the n-side electrode 107 is not constant, but the current from the p-side electrode 106′ may be concentrated at the short side center between the p-side electrode 106′ and the n-side electrode 107 as shown by a thick arrow in FIG. 2. Part of the current flowing from the p-side electrode 106′ to the n-side electrode 107 may flow via the short side center and sideward. This part of the current cannot sufficiently reach the end of the transparent electrode layer 105 near the n-side electrode 107 as shown by the thick arrow and thin allows in FIG. 2 if the distance between the p-side electrode 106′ and the n-side electrode 107 is larger than a current diffusion distance D, which will be described later. In this case, a weak light emission region 108 may be generated where a light emission strength is remarkably low compared with the surrounding region near the p-side electrode 106′, resulting in uneven light emission distribution. As shown in FIG. 2, in order to cause a current to flow to both outer sides in the short side direction, both the p-side electrode 106′ and the n-side electrode 107 should be configured to occupy sufficient areas on both outer sides in the short side direction. However, in this case, the light extracting surface of the optical semiconductor device may be small. Namely, since the occupying ratio of the p-side electrode 106′ with respect to the p-type semiconductor layer 104 is large, the p-side electrode 106′ may shield the light emission from the active layer 103, thereby degrading the light extraction efficiency or light output.
FIG. 3 shows one countermeasure for solving the above problem. Specifically, the active layer 103, the p-type semiconductor layer 104, and the transparent electrode layer 105 are partly removed at the center area to configure the active layer 103′, the p-type semiconductor layer 104′, and the transparent electrode layer 105′, thereby avoiding concentration of current at the center area (see FIG. 5 of Japanese Patent Application Laid-Open No. 2008-4729).
The optical semiconductor device in FIG. 3 is configured such that the p-type semiconductor layer and the active layer are partly removed to form the p-type semiconductor layer 104′ and the active layer 103′, thereby improving the unevenness of light emission distribution to some extent by causing the current to flow in the short side direction. However, the area of the active layer per one optical semiconductor device, or the light emission region, is decreased by the removed active layer, thereby lowering the light output. Furthermore, the distance between the p-side electrode 106′ and the n-side electrode 107 is still not constant. Although the current flow from the p-side electrode 106′ to the n-side electrode 107 can be improved when compared to the case of FIG. 2, the current flowing sideward in the short side direction may not reach the end of the transparent electrode layer 105′ near the n-side electrode 107 as shown in FIG. 3 by thin arrows if the distance between the p-side electrode 106′ and the n-side electrode 107 is greater than the current diffusion distance D which will be described later, thereby generating weak light emission regions 108a and 108b. Accordingly, the light emission distribution becomes uneven.
FIGS. 4A and 4B show another face-up optical semiconductor device as a comparative example where a rectangular face-up optical semiconductor device similar to one shown in FIG. 2 is provided with a p-side electrode composed of a pad portion and auxiliary electrode portions. FIG. 4A is a top view and FIG. 4B is a cross sectional view taken along B-B line in FIG. 4A.
In FIGS. 4A and 4B, respective semiconductor layers including an n-type GaN layer 2, an active layer 3, and a p-type GaN layer 4 are formed on a C-plane sapphire substrate 1. Then, part of the p-type GaN layer 4 and part of the active layer 3 are removed, and a transparent electrode layer 5 is formed over all or most of the entire surface of the remaining p-type GaN layer 4. Furthermore, a p-side electrode 6 including a pad portion 61 and auxiliary electrode portions 62a and 62b is formed on the transparent electrode layer 5. Then, an n-side electrode 7 is formed on the exposed portion of the n-type GaN layer 2.
In FIGS. 4A and 4B, the face-up optical semiconductor device has a rectangular plan shape having a longitudinal width of approximately 510 μm and a short side width of approximately 310 μm. The pad portion 61 of the p-side electrode 6 and the n-side electrode 7 can have a circular shape of approximately 60 μm in diameter so as to facilitate the wire bonding process. The auxiliary electrode portions 62a and 62b are formed on a circle with the n-side electrode 7 as a center. This means that the distance between the auxiliary electrode portions 62a and 62b of the p-side electrode 6 and the n-side electrode 7 can be made almost constant. In this semiconductor device configured as described above, the current flowing between the p-side electrode 6 and the n-side electrode 7 can be evenly diffused, and at the same time the p-side electrode 6 (or pad portion 61) can be decreased in area by the auxiliary electrode portions 62a and 62b. As a result, the ratio of the area of the p-side electrode 6 with respect to the light extracting surface can be decreased, thereby increasing the light output.
However, the conventional optical semiconductor device shown in FIGS. 4A and 4B has the following drawbacks, which will be described with reference to FIG. 5.
Supposing a case where the electrical resistivity is constant in the device, a current can flow through a shortest path between the p-side electrode 6 and the n-side electrode 7. In this case, since the distance between the auxiliary electrode portions 62a and 62b of the p-side electrode 6 and the n-side electrode 7 is made almost constant, the current can be evenly diffused between the p-side electrode 6 and the n-side electrode 7 as shown by solid lines and dotted arrows in FIG. 5. However, it is difficult to cause current to flow through regions 201a and 201b that are not interposed between the p-side electrode 6 and the n-side electrode 7, and accordingly, light emission from that region becomes weak (the regions may be referred to as “weak light emission regions”), whereby the light emission distribution becomes uneven.
To take a countermeasure, it can be considered to extend the auxiliary electrode portions 62a and 62b to the weak light emission regions 201a and 201b. However, in this case the longitudinal size of the optical semiconductor device should be expanded, meaning that the distance between the p-side electrode 6 and the n-side electrode 7 may not be constant. This may result in generation of current concentration region, which are not desirable.
Meanwhile, a current diffusion state within the transparent electrode layer 5 can be determined by a sheet resistance per unit length of the transparent electrode layer 5 and the contact resistivity between the transparent electrode layer 5 and the semiconductor layer (in this case, the p-type GaN layer 4). FIG. 5 shows the current flowing state where the solid arrows represent the main stream of current through any of the transparent electrode layer 5, the p-type GaN layer 4, and the active layer 3, and the dotted arrows represent the main stream of current through the n-type GaN layer 2 via the active layer 3. As shown in FIG. 5, almost all the current from the p-side electrode 6 cannot reach the end of the transparent electrode layer 5 near the n-side electrode 7 and can flow through the n-type GaN layer 2 at the region 202 below the active layer 3. The current scarcely flows through the transparent electrode 5 at the region 202, and accordingly, also scarcely flows through the underlying active layer 3, meaning that the underlying active layer 3 has a very low current density. As a result, the region 202 becomes a weak light emission region.
In the comparative example shown in FIGS. 4A and 4B, there are the large weak light emission regions 201a, 201b, and 202, thereby making the light emission distribution uneven, which is a problem.
It should be noted that an ideal current density of an optical semiconductor device with a completely even light emission distribution can show a constant value all over the transparent electrode layer. Namely, this means that the difference in light emission intensity between the most strong light emission region and the most weak light emission region in the optical semiconductor device, i.e., the difference in current density J (d) is small. However, the face-up optical semiconductor device shown in FIGS. 4A and 4B has the p-side electrode and the n-side electrode 7 that are disposed away from the semiconductor layers (2, 3, and 4) in the vertical direction (stacked direction) and the horizontal direction (plane direction of the device). This may result in uneven diffusion of current sufficiently to the end portion of the optical semiconductor device, i.e., the n-side electrode 7. As a result, the weak light emission regions 201a, 201b, and 202 cause an uneven light distribution.
The following three methods may be considered for decreasing the light emission intensity difference in the optical semiconductor device to decrease the weak light emission region 202, namely, to increase the current diffusion distance D.
One of the methods is such that the thickness of the transparent electrode layer 5 can be increased to decrease the sheet resistance ρITO. When the sheet resistance can be decreased, the current density J (d) can be lowered at d=0 whereas the current density J (d) at the n-side electrode 7 can be increased, thereby obtaining an ideal current density J (d). However, this method may have adverse effects such that, when the light passes through the transparent electrode layer 5, the amount of absorbed light may increase to lower the light extraction efficiency. Furthermore, decreasing the sheet resistance lower than 5 Ω/sq. is quite difficult in terms of manufacturing. In addition to this, the deposition process may require a longer time, thereby decreasing the yield in view of maintaining a uniform film thickness in the wafer and also increasing the manufacturing cost.
The second method is to increase the contact resistivity r2 between the transparent electrode layer 5 and the semiconductor layer (in this case, it can be p-type GaN layer 4). However, this method may increase the forward voltage in the optical semiconductor device.
The third method is to saturate the current flowing into the active layer 3 by increasing the supplied current IS, thereby increasing the current diffusion distance D. In this method, however, the bonding wire connected to the pad portion 61 and the thin auxiliary electrode portions 62a and 62b may be supplied with a large amount of current. This may result in reliability deterioration, such as disconnection of bonding wires, peeling off of the auxiliary electrode portions 62a and 62b, and the like, and decreased inner quantum efficiency may arise due to the increased current density.